The present invention relates to a system and apparatus for dewatering a suspension of solids in a carrier liquid by means of an electrically augmented vacuum filtration (EAVF) system, wherein an anodic electrode structure comprises a non ion selective membrane which is permeable to both cations and anions, but which is impervious to the suspension of solids, carrier liquid, and electrolyte. The non ion selective membrane according to the present invention comprises a substrate and a solid polymeric film, wherein the film comprises a cation exchange resin, an anion exchange resin and a binder.
The use of an EAVF system for dewatering a suspension of solids in a carrier liquid is described in U.S. Pat. Nos. 4,168,222 and 4,207,158. These patents describe a means for dewatering a suspension of solids in an electric field which is controllably maintained between opposing electrodes, to cause the solids to migrate relative to the carrier liquid to form a layer or cake on one set of electrode structures in which the electrode elements are positioned within ion-pervious walls and immersed in a selected electrolyte, while allowing carrier liquid to be withdrawn under vacuum in the opposite direction through the liquid-pervious wall of a hollow, counter electrode structure, and wherein the layer or cake material maybe detached from the first electrode structure during exposure from the suspension. The ion-pervious wall of the counter electrode structure in U.S. Pat. No. 4,168,222 comprises a chemically and electrically neutral filter media or permeable porous membrane which, the film-like nature or otherwise requiring support, maybe backed by a chemically and electrically neutral grid so that a planar electrode filter surface is presented to the slurry being treated. Furthermore, a vacuum is applied internally to the counter electrode to reduce the pressure on the filter media when the electrode is in the raised position for doctoring of the cake of solids formed thereon, whereby the vacuum prevents the loss of electrolytes and/or rupture of the filter media.
U.S. Pat. No. 4,362,612 also discloses the use of an ion non-selective permeable membrane which is directed to an apparatus for isoelectric focusing, wherein a plurality of ion non-selective permeable membranes are used to streamline the flow of liquid while allowing interchange of fluid constituents there-between. These membranes allow interchange of fluid constituents there-between while providing the desired streamlining. Thus, the ion non-selective permeable membranes according to U.S. Pat. No. 4,362,612 is liquid permeable which would result in loss of electrolyte from the electrode structure similar to U.S. Pat. No. 4,168,222 which is undesirable.
It is a disadvantage of U.S. Pat. Nos. 4,168,222, 4,207,158 and 4,362,612 that the ion pervious wall or ion non-selective permeable membrane permits the loss of electrolyte from the anodic electrode structure. The electrolyte which passes through the ion-pervious wall has a detrimental effect on both the cake formed on the ion-pervious wall of the anodic electrode structure and the filtrate or carrier liquid of the bath.
It has also been known to use ion permeable membranes in electrodialysis as apparently described in U.S. Pat. No. 3,510,417, wherein the ion permeable membranes can selectively effect the permeation of the ions of a smaller valance from among those having the same charge. The ion exchange membrane for selectively transfering ions of the same charge in accordance with U.S. Pat. No. 3,510,417 are composed of insoluble, infusable synthetic organic polymers having an ionic group chemically bonded thereto. Accordingly, the ion permeable membrane can be either a cationic exchange membrane, those having active acidic functional groups such as --COOH bonded to the polymer, matrix or an anionic exchange membrane, those having a nitrogen-containing active based group such as quateranry ammonium, amino group, quanidyl group, and dicyandiamidine group bonded to the polymer matrix.
Perm-selective membranes have recently been used in EAVF systems as apparently described in U.S. Pat. No. 4,312,729 (Wills). Wills appears to describe that a perm-selective membrane can be used in an electrokinetic cell of an electrically augmented vacuum filter to prevent a migration of anions from the anode and optionally cathode compartments. The perm-selective membrane in the form of a film laminated to a support grid effectively prevents migration of anions into the cake filter and migration of fine particles solids to be filtered from the slurry into the anolyte and/or catholyte (Also see U.S. Pat. No. 4,419,209).
It is a disadvantage of U.S. Pat. Nos. 3,510,417, 4,312,729 and 4,419,209 in that the membranes are ion selective. That is, they teach the use of either an anion selective membrane or cation selective membrane which result in the following problems when such membranes are used in an EAVF system.
When, for example, an ion selective membrane is used at the anode in an EAVF system for dewatering a suspension of solids in a carrier liquid certain disadvantages occur. If a cationic ion selective membrane is used at the anode then the cake of suspension solids formed on the membrane has a significant increase in soluble ion concentration due to the impermeability of the membranes to anions which are attracted towards the anode, and the permeability of the membranes to cations which pass from the anolyte into the cake to satisfy charge neutrality. Conversely, if an anionic selective membrane is used at the anode then the cake of suspension solids formed on the membrane is depleted of cations.
In U.S. Pat. No. 4,312,729 (Wills) it is proposed that an ion selective membrane be placed around the anodic electrode to prevent the migration of anions from the anode into the filter cake and the migration of fine particles solids to be filtered from the slurry into the anolyte. In this instance reverse dialyzing occurs wherein anions are attracted toward the anode and pass through the cake formed on the cation selective membrane but which are unable to pass through the cation selective membrane and become trapped in the cake thus causing anion build-up in the cake whereby the cake becomes contaminated while the pH is moderate. Although a moderate pH is desired, the build-up of cations in the cake results in a contaminated precipitate which is not desired.
If we were to substitute an anion exchange membrane as disclosed in U.S. Pat. No. 3,510,417 for the cation selective membrane of Wills at the anode, this would result in dialyzing of the filter cake. That is, although the anions which are attracted to the anode would pass through both the cake and the anion selective membrane, the cations in the anolyte solution would be trapped in the anolyte since they are impervious at the anion selective membrane so that most of the cations would be extracted from the cake toward the cathode ultimately causing a decrease in the pH of the cake due to the polarization effects adjacent to the membrane. If the pH of the cake is substantially reduced, as is the case when an anion selective membrane is used at the anode then the cake becomes flocculated which is undesirable.
The present invention overcomes the problems of both dialyzing and reverse dialyzing of the cake formed at the anode membrane so that a cake of suspension solids is formed which is extremely desirable, i.e., not contaminated and of moderate pH. Furthermore, the present invention overcomes the disadvantage of allowing electrolyte to diffuse or convect through the ion-pervious walls of the prior art. The advantages of the non ion seletive membrane of the present invention will be further described below.